The present invention generally applies to linear signal amplification, and particularly relates to using inner and outer feedback loops in a multi-stage amplifier.
Linear signal amplification broadly refers to generating one signal that is directly proportional to another signal, but with some desired gain (or attenuation) in signal amplitude or power. A simple example is generating an output sine wave having twice the amplitude of an input sine wave. Of course, one understands that practical applications of linear signal amplification extend into considerably more complex and challenging examples.
Characteristically, linear signal amplification uses a closed-loop amplifier approach wherein a negative feedback loop is closed around the amplifier. That is, the amplifier output signal is fed back in inverting fashion to an input of the amplifier. The benefits of negative feedback are many and include reduction of output error, reduction of sensitivity to component or device parameters, predictability of gain over frequency, and reduced sensitivity to disturbances.
Despite the benefits of negative feedback in linear amplifiers, it can be problematic in terms of amplifier stability, for example. Stability issues arise for a number of reasons, but generally involve the relative phase of the input and feedback signals. For example, the ubiquitous operational amplifier integrated circuit finds common use in linear amplification applications. Essentially all operational amplifiers have a low-frequency pole that introduces a ninety-degree phase shift in their output signals beyond a certain frequency. As amplifier signal frequencies increase, the phase shift between input and output signals, and, therefore, between input and feedback signals, tends to increase. At some point, the relative phasing transitions through 180 degrees and negative feedback become positive feedback, which transforms the erstwhile amplifier into an oscillator.
At the frequency where the phase equals 180 degrees, the loop-gain must be substantially less than 1 to insure stability. The loop-gain is defined as the open-loop gain divided by the closed-loop gain. Phase margin is defined as the difference between 180 degrees and the phase value at the frequency where the loop-gain passes through a value of 1. A phase value substantially less than 180 degrees helps to insure stability, but a loop-gain substantially greater than 1 helps to insure accuracy and linearity. Consequently, tradeoffs between loop-gain and phase margin are usually inevitable.
Certain types of loads, and even amplifier structures, exacerbate potential stability problems. For example, capacitive loads introduce additional phase shift and further reduce amplifier phase margin, where phase margin connotes the amount of additional phase lag the amplifier can tolerate before becoming unstable. As input signal frequencies increase, potential instability problems also increase. For example, lead and frame inductances in integrated circuit devices come into play, as do trace inductances in the physical circuit boards. Further, small capacitances, such as MOSFET gate-to-drain capacitance, come into play at higher signal frequencies. In short, linear amplifier design becomes decidedly more challenging as signal frequencies increase.
Radio frequency (RF) amplifier design is one area in particular that is rich in linear amplifier design challenges. Here, the signals of interest easily extend into the tens of MHz, and oftentimes extend into the GHz range. Further complicating these design challenges, RF amplifiers are often required to provide significant output power. This power requirement and other needs drive RF amplifier design towards multi-stage amplifier implementations that use a high-current output stage often comprising an AB-class MOSFET amplifier that itself has significant capacitive loading effects on prior amplifier stages.
Indeed, multistage amplifiers, whether or not in the context of RF signal amplification, in general pose significant design challenges in terms of bandwidth capability and stability. Phase margins are typically poorer due to the greater cumulative phase shift of the multi-stage signal path. In some instances, the overall phase shift of multi-stage amplifiers is such that closing the feedback loop from the final stage output to the initial stage input invites instability rather than preventing it.
What is needed then is an approach to multi-stage amplifier design that allows designers to implement full bandwidth multistage linear amplifiers that realize the benefits of negative feedback without the attendant problems it normally introduces in such applications. Preferably, the approach would not introduce overly complex design requirements, and would be practical in terms of cost and simplicity of physical implementation.